This disclosure relates to polycarbonates comprising aliphatic diols, and in particular to isosorbide-based polycarbonates, and methods of manufacture thereof.
Polycarbonate homopolymers and copolymers of aliphatic diols, particularly isosorbides (i.e., 2,6-dioxabicyclo[3.3.0]octan-4,8-diol and isomers), are of great interest to the chemical industry because such aliphatic diols can be produced from renewable resources, namely sugars, rather than from petroleum feed stocks as for most presently used bisphenol monomers.
However, the homo- or copolycarbonate incorporating isosorbide needs to be of sufficiently high molecular weight to have properties of practical importance. There have been several previous attempts to produce polycarbonates from isosorbide, but each of these attempts has had its difficulties, and therefore at present, such polycarbonates are not produced commercially. For instance, the primary method for preparing polycarbonates generally uses interfacial polymerization in methylene chloride/water using phosgene and alkali. In this method, one or more diols (e.g., bisphenols) in an aqueous alkaline solution are mixed thoroughly, with stiffing, in methylene chloride or other suitable halogenated solvent and can be converted to high molecular weight polycarbonates by introducing phosgene. However, this method is not suitable for preparing homopolycarbonate derived from isosorbide because the isosorbide solubility in water is too high, impeding the interphase transfer, and its acidity too low to proceed at an adequate rate in pH ranges suitable for interfacial phosgenation.
Isosorbide polycarbonate homopolymer has been prepared by solution polycondensation in pyridine containing solvent mixtures at low temperatures. It has also been prepared by converting isosorbide to the bischloroformate and polymerized by interfacial polymerization. The polymer obtained exhibited a Tg of 144 to 155° C. (See e.g., Angew. Makromol. Chem., 1993, vol. 199, p. 191; U.S. Pat. No. 4,506,066; Macromolecules 1996, vol. 29, p. 8077). Attempts have also been made to prepare copolycarbonates of isosorbide with bisphenol A by interfacial method in an alkaline water/methylene chloride mixture with phosgene. Only bisphenol A polycarbonate was obtained and no incorporation of isosorbide was observed (U.S. Pat. No. 4,506,066).
An alternative method of synthesizing polycarbonates is by use of melt polymerization. The reaction of a bisphenol with a source of carbonate units such as diphenyl carbonate (DPC) in the presence of a catalyst and the absence of solvent are typical of melt polymerization method. A first attempt to prepare isosorbide polycarbonate by melt transesterification reaction with DPC was reported in 1967. The polycondensation was carried out without the use of catalyst at 221° C. and the pressure was reduced from atmospheric pressure to 1 mm Hg. (Great Britain U.S. Pat. No. 1,079,686). A brown white powder containing higher melting and cross-linked constituents was obtained. Further attempts to prepare copolycarbonates of isosorbide with BPA, 4,4′-dihydroxydiphenyl sulfide, and 4,4′-dihydroxy biphenyl by condensation with DPC at a temperature up to 200° C. using disodium salt of bisphenol A as transesterification catalyst was carried out and resulted in the formation of oligocarbonates. The phenylcarbonate end groups of the oligocarbonates were hydrolyzed and polymerized by interfacial polymerization to high molecular weight copolycarbonates. (German Patent No. OS 3,002,276). The melt polycondensation approach was repeated in 1981 (U.S. Pat. No. 4,506,066), wherein isosorbide was condensed with DPC at 220° C. and a pale brown polymer along with insoluble constituents was obtained. In this study, it was presumed that during melt polymerization conditions branching had occurred and leads to the formation of insoluble inhomogeneous product. Thus, it was concluded that melt polycondensation is not suitable for the preparation of isosorbide homo- and copolycarbonates. Further evidence to this comes from the detailed polymerization work carried out by Kricheldorf et. al. (Macromolecules 1996, vol. 29, p. 8077). One-step polycondensation of isosorbide diphenyl carbonate with various diphenols catalyzed by ZnO was carried out. This study also led to the formation of product which was insoluble in all common solvents tested.
U.S. Pat. No. 7,138,479 disclosed an activated carbonate melt process to synthesize isosorbide based copolycarbonate which had random arrangements of structural units. However, the attempts to synthesize isosorbide homopolycarbonate using this method only resulted in relatively low molecular weight isosorbide carbonate homopolymer. Examples 1 and 2 in Table 2 of U.S. Pat. No. 7,138,479 disclosed Mw values (gel permeation chromatography, polystyrene standards) of 16,060 g/mol and 20,678 g/mol, prepared using non-activated and activated melt polymerization processes, respectively; however, such Mw values are not sufficiently high for practical use in most, if not all, commercial applications.